This invention relates to a method and apparatus for obtaining an X-ray image of teeth. Specifically, the method and apparatus use an intra-oral (i.e. in-the-mouth) linear array sensor to detect radiation that originates from a radiation source outside the mouth, and create an image from the output of the sensor.
The procedure most commonly used by dentists to obtain radiographic images of the teeth, jaw structure or the like of patients produces what is called a periapical X-ray radiograph. In this known procedure, an X-ray film packet is inserted into the patient's mouth near the teeth or other anatomical structure to be imaged. The procedure makes use of an X-ray tube of the type which generates X-rays at a fixed point on an anode and in which the X-rays radiate out from that fixed point. The tube is provided with a shield cone which is directed at the film packet through the teeth or other structures to be imaged. Precise positioning of both the film packet and the shield cone is necessary to obtain useful X-ray images.
Although the conventional periapical X-ray procedure is very extensively used, it is subject to several serious disadvantages. The need to insert and retain a relatively large film packet in a patient's mouth, for example, often causes discomfort or gagging and may not be tolerable to certain patients such as small children and elderly persons. Further, no visible image is available to the dentist until the film packet has been removed and subjected to a time-consuming and costly development procedure. An instantaneous radiographic image would be much more useful to the dentist. In addition, this development procedure requires the use of chemicals which can have a negative environmental impact.
Another very serious problem with film is the undesirably large radiation dosage that is needed in order to produce a complete set of dental X-ray images. This is in part a result of the very low detection efficiency of the unscreened X-ray film commonly used for dental X-ray operations.
Techniques for reducing radiation exposure have heretofore been developed utilizing screened film that greatly improves detection efficiency by disposing an image-intensifying phosphorescent material on the X-ray film emulsion surface. The phosphorescent screen, however, reduces the definition of the resulting images, and has accordingly proved to be impractical in many situations.
The problem of high radiation exposure in dental radiology is often aggravated by a need to repeat the X-ray imaging process. For example, after the X-ray film is developed, it may be found that the film was not properly aligned during the original exposure, or that errors were made in developing the exposed X-ray film. Each of these occurrences is fairly common.
Some newer X-ray imaging systems do not use photographic film, and thereby avoid some of the disadvantages described above. For example, U.S. Pat. No. 4,160,997 (Schwartz), the pioneer patent in the field of filmless dental radiography, describes a system for taking periapical X-rays using an electronic sensor instead of photographic film. In Schwartz, X-rays which have passed through the patient's teeth are converted into light by a phosphorescent screen, and the light is detected by a two-dimensional solid state image pickup device such as a CCD (charge coupled device). The screen and the image pickup device are packaged together and connected to an electronic video display system. Improvements to this type of system are described in U.S. Pat. No. 5,434,418 (Schick).
Systems of the type described in Schwartz address some of the problems associated with conventional periapical X-ray systems described above. In particular, because electronic sensors that use phosphorescent screens are much more sensitive to X-ray radiation than photographic film, systems using these sensors can use a much smaller dosage of X-rays to register an image. In addition, because the output of the electronic sensor can be processed very rapidly by a micro-computer, the images are available immediately after the exposure is taken. Finally, no film developing chemicals are needed with these electronic sensor systems.
However, while the electronic sensor systems provide significant advantages over traditional photographic periapical systems, some disadvantages remain. Most notably, it is still necessary to insert the electronic sensor into the patient's mouth repeatedly, in various positions, and to take multiple exposures.
Because a full set of periapical X-rays includes up to 21 X-ray pictures, the process of asking the patient to open his mouth, inserting the film packet or electronic sensor, walking out of the room, actuating the X-ray tube, returning to the room, and repositioning the sensor must be repeated numerous times. Even when an abbreviated set of "bite wing" periapical X-rays are taken, the process must be repeated four times. This takes a significant amount of time, even when performed by a highly skilled technician, and can be even more time-consuming with a less experienced technician. In addition, the process is uncomfortable for the patient because biting down on the sensor package can be painful, and because the patient cannot speak for extended periods of time.
The panoramic (also known as pantomographic) image technique is another advance over standard periapical X-ray imaging, and has been extensively utilized in the recent past by dentists and oral surgeons. In this procedure, panoramic or wide angle X-ray images are produced by generating a narrow linear X-ray beam which is revolved during the exposure about an axis of rotation which passes through the anatomical structures to be examined. The X-ray tube is essentially a conventional one which generates X-rays at a small fixed point on an anode. Radiation generated at this point is collimated by a first slit which is parallel to the axis of rotation, passes through the patient's head and then through a second similar collimating slit situated in front of a screened film cassette which is rotated in synchronism with the rotational movement of the X-ray beam. The tube and detector motion causes the X-ray beam to sweep across the intervening anatomical structures. Upon development of the film, a panoramic two-dimensional strip image is produced of curved anatomical structures in the patient's head such as the mandible or maxilla.
Although a significant reduction of patient radiation dosage may be realized in comparison with periapical procedures, the conventional panoramic image technique is itself subject to several disadvantages. It is necessary that the X-ray beam pass through the entire skull of the patient, even if it is only desired to obtain an image of a portion of the skull such as the dental arch. Consequently unwanted images are superimposed upon the desired image data. This makes interpretation of the image more difficult and detracts from the general quality of the image by obscuring desired data to some extent with undesired information. Moreover, radiation exposure remains undesirably high as the X-ray beam must necessarily pass through the entire skull. Anatomical structures which are not of particular interest are thereby necessarily subjected to radiation dosage which does not contribute any useful information but actually detracts from the quality of the desired data. Further, a significant amount of X-ray scattering occurs during passage of the X-ray beam through the patient's entire head, creating a background fog in the image on the developed film that undesirably limits the range of contrast in the image and which may cause loss of definition. This process also requires the time consuming and troublesome film developing process of the conventional periapical X-ray procedure.
Additional losses of definition and contrast arise from the presence of the intensifying screen in front of the X-ray film. Underlying and supplementing these contrast limitations peculiar to the panoramic image technique is the undesirably limited grey scale latitude of X-ray film in general. Still further, a long exposure time, typically about 20 seconds, is needed to complete a full-mouth panoramic image. As result, problems often arise from patient motion or equipment vibration with consequent blurring of the resulting X-ray images. This tends to be particularly severe when the patient is an infant or young child. Finally, a considerable degree of distortion of the depicted objects is normally present in the conventional panoramic image.
Attempts have been made to develop a system that eliminates some of these shortcomings. U.S. Pat. No. 4,323,779 (Albert) describes a panoramic X-ray system that uses an electronic sensor instead of photographic film. In particular, Albert teaches the use of a sweeping electron beam to produce a moving point source of X-rays. The X-rays from this moving point source pass through the patient's teeth and strike an X-ray detector, which is made of a small scintillator crystal that converts the X-rays into visible light and a photosensitive element that converts the light into an electrical signal. As the point source is moved, the X-rays from the source will pass through different points on the patient's teeth on their way to the sensor. A portion of the X-rays is absorbed by the teeth, and the rest strikes the sensor. The electrical signal produced by the sensor is proportional to the portion of X-ray radiation that arrives at the sensor. This electrical signal is then processed by appropriate electronic circuitry.
Albert's system, however, has several drawbacks of its own. In particular, the system relies on complex components, including an electron gun, an anode plate, and a deflector means for moving the point X-ray source. Two problems are inherent in this type of system: Optical distortions must be compensated for, and focal length adjustments are often required. Moreover, the Albert system cannot provide an panoramic image of the entire mouth in one operation--it can only obtain an image of a portion of the dental arch. Therefore, multiple exposures are required in order to obtain a full panoramic X-ray image of the patient's teeth. Moreover, attempts to increase the number of teeth in a single image result in increased optical distortion.
There is therefore a need to develop a substantially improved dental radiography system that departs from past approaches.